The present invention relates to a method for manufacturing a dispenser cathode structure, and more particularly to a method for manufacturing a dispenser cathode structure wherein a porous pellet is impregnated with thermoelectron-emitting material.
Generally, dispenser cathodes are classified into a cavity reservoir type, an impregnated type, and a sintered type according to their structure. A common characteristic of these is that, since their beam current density can be heightened, they are suitable for a full-sized Braun or projecting tube, and especially, are very durable.
FIG. 1 is a sectional view showing one example of a conventional impregnated-type dispenser cathode structure. This cathode structure includes a porous pellet 2 formed of a refractory metal selected from a group consisting of tungsten, molybdenum, etc., sintered with an electron emitting material, a reservoir 3 for accommodating pellets 2, and a sleeve 4 which includes a heater 4a and supports reservoir 3. A metal covering layer 5 formed of at least any one of Ir, Os, Ru, Sc, etc., is provided on the surface of pellets 2 supported by reservoir 4. This metal covering layer 5 forms an alloy in conjunction with a metal on the surface of pellet 2.
A method for manufacturing impregnated cathode structure 1 formed as above is described below. 1. A metal powder such as molybdenum, tungsten, etc., is press-molded into a predetermined shape, and fired to produce pellets 2 which are then inserted into cup-shaped reservoir 3 formed of a heat-resistant material, thereby welding them to each other.
2. Electron emitting material 2a composed of mixed BaO, CaO, and Al.sub.2 O.sub.3 is melted and impregnated into pellets 2 within a vacuum heating furnace at 1500.degree.-1700.degree. C. or a heating furnace in a hydrogen gas ambient.
3. Residue 2a' having adhered to the surfaces of pellet 2 and reservoir 3 during the impregnating process, are eliminated by an abrasive polishing of those surfaces.
4. Metal covering layer 5 composed of at least any one of Ir, Os, Ru, Sc, etc., is formed on the surface of pellets 2 by a sputtering method.
5. Reservoir 3 is inserted into the upper portion of separately formed cylindrical sleeve 4, followed by welding them to each other.
6. Finally, a heater is housed in the sleeve, during its assembly into an electron gun.
In the method for manufacturing the conventional impregnated cathode as described above, fine sandpaper is utilized in the process for eliminating the residue of the impregnation having adhered to the surface of pellets 2 and reservoir 3. Under certain circumstances, grit-blasting is employed, wherein hard, minute particles such as Al.sub.2 O.sub.3 powder and the like are sprayed at high speed at the residue adherent to pellets 2 and reservoir 3, so that the residue is eliminated by the impact of the minute particles. In the abrasion method using the sandpaper or grit-blasting, the surface of pellets 2 gets stripped off or becomes deformed, so that the pores are partially clogged as shown in FIG. 2B, which impedes the diffusion of the cathode material over the surface of the pellet. Moreover, due to inconsistent abrading, the residue of the impregnation remains on portions of the surface of pellet 2.
FIG. 3 illustrates the measurement of the surface of a pellet having the residue eliminated using the above-described conventional techniques, by electro-probe microanalysis (EPMA). Here, the highest peak indicates the presence of tungsten, and the two smaller peaks show the presence of calcium and barium. Like this, the electron emitting materials remaining on the pellet produce other impurities by reacting with CO.sub.2, H.sub.2 O, and the like which are included in air. Especially, the electron emitting material expands in size by reacting with the H.sub.2 O, thereby partially covering or damaging metal covering layer 5 on the surface of the pellet. In addition, according to such abrasive polishing methods, other impurities are introduced from the sandpaper or abrasive particles during the elimination of the residue of impregnation, which may cause more serious problems.
On the other hand, in order to solve the problems with the abrasive polishing methods, ultrasonic waves are applied to a clearing water in which the pellet impregnated with the cathode materials is soaked, so that the impregnated residue on the surface of the pellet is eliminated by ultrasonic vibrations of the cleaning water (refer to U.S. Pat. No. 4,417,173 and Applications of Surface Science 8, pp. 13-35, North-Holland Publishing Company, 1981). However, according to this ultrasonic cleansing method, not only the unnecessary residue adherent on the pellet is eliminated but also a certain amount of cathode material present in the inner cavities of the pellet. Such loss of cathode material serves to reduce the cathode's ability to emit thermoelectrons.